Aha! It looks like State Farm's app isn't the only app that helps keep you from texting while driving - there's SMS Replier too. And this app will know when you're driving.
So not only can you set customised replies, you can also choose how long you're doing the activity, so the phone will only reply "In a meeting" during the two hours that you set that you're in a meeting. (Would be nice if this synced up with your Google Calendar too, to automatically know what you're doing.) What's also cool is if someone calls you during this time, SMS Replier will auto-reply to that as well, saying you'll call them back when you're done.
Other features: Up to five numbers that you can add to get incoming calls and texts. Say, for emergency calls. Or also set numbers that you don't auto-reply to.
The app's available for Android, BlackBerry and Windows Mobile. [SMS Replier]